Eoin,
As many will tell you, the PPL is a licence to learn. What you have is a normal worry, felt by all apart from those people who are overconfident. One of those, after getting his PPL in minimum hours, flew off by himself and landed at Llanbedr thinking it was Caernarfon.
And while landing at the wrong airfield has been done many times, two airfields a long way apart, on very different looking stretches of coastline, with very different runway layouts....well, him I'd worry about, you I wouldn't. You are right; you still have loads to learn. We all do. The examiner obviously thought you knew enough to be capable of learning more by yourself, that's all.
When I was doing my PPL(A), I kept getting things wrong, and I seemed to have to repeat every exercise, including navs. On my second solo cross country, I got well and truly lost. When I finally got back, my instructor, normally a very mild and calm man, had steam coming out of his ears. How, he wanted to know, had I got lost on a 16 minute leg, on a route I'd flown before, and mistaken one town for another that didn't look anything like it? "Come here and I'll show you", I said, having spent a long time analysing it all myself. I explained, and then he said: "OK, you'd better book your NFT (Navigation Flight Test; this was pre-JAA days)". I said in astonishment: "Don't you want me to repeat that nav?" He said there was no point; I knew where I'd gone wrong and why, and I'd learned from it, which was all that was required; repeating it would achieve nothing.
I'm not in your examiner's head, but no doubt he felt similarly. Trust him, and yourself!